This invention relates to the manufacture of modules for non-cryogenic separation of gases into components. In particular, this invention provides an improved method of manufacturing a module containing a bundle of polymeric fiber membranes. More specifically, the present invention concerns the formation of tubesheets at both ends of the gas-separation module.
It has been known to use a polymeric membrane to separate air into components. Various polymers have the property that they allow different gases to flow through, or permeate, the membrane, at different rates. A polymer used in air separation, for example, will pass oxygen and nitrogen at different rates. The gas that preferentially flows through the membrane wall is called the “permeate” gas, and the gas that tends not to flow through the membrane is called the “non-permeate” gas. The selectivity of the membrane is a measure of the degree to which the membrane allows one component, but not the other, to pass through.
A membrane-based gas separation system has the inherent advantage that the system does not require the transportation, storage, and handling of cryogenic liquids. Also, a membrane system requires relatively little energy. The membrane itself has no moving parts; the only moving part in the overall membrane system is usually the compressor which provides the gas to be fed to the membrane.
A gas separation membrane unit is typically provided in the form of a module containing a large number of small, hollow fibers made of the selected polymeric membrane material. The module is generally cylindrical, and terminates in a pair of tubesheets which anchor the hollow fibers. The tubesheets are impervious to gas. The fibers are mounted so as to extend through the tubesheets, so that gas flowing through the interior of the fibers (known in the art as the bore side) can effectively bypass the tubesheets. But gas flowing in the region external to the fibers (known as the shell side) cannot pass through the tubesheets.
In operation, a gas is introduced into a membrane module, the gas being directed to flow through the bore side of the fibers. One component of the gas permeates through the fiber walls, and emerges on the shell side of the fibers, while the other, non-permeate, component tends to flow straight through the bores of the fibers. The non-permeate component comprises a product stream that emerges from the bore sides of the fibers at the outlet end of the module.
Examples of fiber membrane modules are given in U.S. patent application Ser. No. 11/137,827, filed May 25, 2005, published under Publication No. 2006-0266217 on Nov. 30, 2006, and U.S. patent application Ser. No. 11/383,301, filed May 15, 2006, the disclosures of both of which are hereby incorporated by reference.
A tubesheet is typically formed over the ends of a bundle of fibers by immersing an end of the bundle in an epoxy, or similar substance. When the epoxy cures, it forms a hard, solid structure which encases the ends of the fibers. This solid structure defines the tubesheet.
In practice, it is desired that the tubesheet be thick enough to anchor the fibers, but not so thick that it occupies too much of the surfaces of the fibers. If the tubesheet covers too much of the surface of the fiber, the utility of the fiber as a gas-separation membrane is impaired, because gas cannot flow through the covered portion. One way of making the tubesheet thicker is to use an epoxy material of relatively low viscosity, so that the material will more readily flow. But if the viscosity is too low, the material may “wick” along the fiber, eventually covering large parts of the fiber and effectively rendering part of the fiber useless. On the other hand, if one uses epoxy mixtures having higher viscosity, the wicking problem is overcome, but it then becomes difficult to obtain a tubesheet of the desired thickness.
The present invention provides a method for making a tubesheet, wherein the tubesheet can be conveniently formed with a relatively high-viscosity material.